Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Qual conta permanente tem registro de convite apenas e userpics gratuitos de desconhecidos?
 ###Tabelas: CREATE TABLE dublin_and_south_eastern_railway_1854–1924 (dublin_and_south_eastern_railway_1854–1924_id AUTO_INCREMENT PRIMARY KEY, Type VARCHAR, Fleet numbers VARCHAR, Quantity made INT, Date made VARCHAR, GSR Class VARCHAR, GSR Nos. VARCHAR, Withdrawn VARCHAR) CREATE TABLE theater_credits (theater_credits_id AUTO_INCREMENT PRIMARY KEY, Year INT, Play VARCHAR, Role VARCHAR, Theatre VARCHAR, Location VARCHAR) CREATE TABLE comparison (comparison_id AUTO_INCREMENT PRIMARY KEY, Name VARCHAR, Mail transfer agent VARCHAR, Mail storage type VARCHAR, IMAP server VARCHAR, IMAP shared VARCHAR, Calendar format VARCHAR, Calendar shared VARCHAR, IM server VARCHAR, WebDAV VARCHAR, Web UI VARCHAR, Contacts shared VARCHAR, Files shared VARCHAR, Web admin UI VARCHAR) CREATE TABLE 2007_2008 (2007_2008_id AUTO_INCREMENT PRIMARY KEY, Date VARCHAR, Site VARCHAR, Sport VARCHAR, Winning team VARCHAR, Series VARCHAR) CREATE TABLE defunct_sites (defunct_sites_id AUTO_INCREMENT PRIMARY KEY, Name VARCHAR, Year Began VARCHAR, Registration VARCHAR, Userpics Free VARCHAR, Userpics Paid VARCHAR, Monthly Cost for Paid Account VARCHAR, Yearly Cost for Paid Account VARCHAR, Permanent Account VARCHAR)  
SELECT Permanent Account FROM defunct_sites WHERE Userpics Free = unknown AND Registration = invite-only